Ultrawideband (UWB) radio is a communications technology which uses short pulses of radiofrequency (RF) energy to transfer data and perform sensing functions. By using very short pulses, UWB signals have very high bandwidth compared to transmissions from traditional radio systems, and this confers advantages in both communicating data at very high speed and performing accurate sensing functions in cluttered environments.
One use of UWB radio technology is in location sensing applications. Typically, tags are attached to the objects to be located, and a network of sensors is placed at known points in the environment. UWB signals emitted by the tags are detected by the sensors, which use them to determine measurements such as the distance from the tag to the sensor. Processing logic can then combine the known sensor positions and the measurements to determine a 2D or 3D position for the tag. Typical UWB location systems are accurate to within a few tens of centimeters, even within environments that are normally challenging for radiolocation systems, for example those with many metal reflective surfaces.
A number of different receiver technologies have been used in sensors for UWB location systems. Generally, they fall into one of two different classes: non-coherent receivers and coherent receivers.
A non-coherent UWB receiver is designed to detect single, individual pulses within a UWB pulse train. Non-coherent receivers can be placed into a ‘waiting’ state, to be triggered only when a UWB pulse arrives at the receiver. At that moment, timing circuitry in the receiver records the time-of-arrival of the pulse.
One form of non-coherent UWB receiver uses a Schottky diode to rectify an incoming pulse signal which has been received via a UWB antenna, and amplified using a low-noise amplifier. The rectified signal is passed through a low-pass filter, and input to a high-speed comparator the latched output of which then triggers timing circuitry in the receiver to record the time-of-arrival of the pulse. The receiver is reset to detect a subsequent pulse by resetting the latch of the comparator.
A non-coherent UWB receiver can be used with an appropriate transmitter to transfer data from the transmitter to the receiver. For example, the transmitter may send a series of on-off keyed (OOK), or pulse-position-modulated (PPM) pulses, which are picked up by the receiver. Since the receiver timestamps the arrival of each pulse from the transmitter, it is able to determine whether a particular pulse from a train with a known period is missing (i.e. it is a ‘0’ bit in an OOK modulation scheme, rather than a ‘1’ bit which would be present), or has been transmitted at a slightly shifted time from that expected in the nominal pulse train (as might be expected if a PPM modulation scheme was used).
Non-coherent UWB receivers are limited in that a sophisticated signal processing analysis of the incoming signal is not typically possible because the signal is only received for a very short period.
A coherent UWB radio receiver is designed to detect a train of broadband radio pulses sent to it by a UWB transmitter. U.S. Pat. No. 5,510,800 describes such a coherent UWB radio receiver for use in position determination applications. In U.S. Pat. No. 5,510,800, the receiver is used to detect the presence or absence of an incoming train of UWB pulses, and measure the time-of-arrival of these pulses. It uses a sampling gate in the receiver, which mixes a replica of the expected incoming pulse with the incoming signal. The mixer has a high output response when it is triggered with a pulse replica at the exact moment when a pulse arrives at the receiver, and a low response if it is triggered when no pulse arrives at the receiver.
The incoming and replica pulses have a width of only a nanosecond or two (so as to generate the wide bandwidth desirable in the UWB system). Generally, the receiver does not initially know at what instant an incoming pulse may arrive at the receiver. It therefore shifts the time at which replica pulses drive the sampling gate across the full pulse repetition period of the pulse train, to ensure that the replica and incoming pulse trains coincide at the receiver at some point. A typical system might have a pulse repetition frequency of 10 MHz or less, so the pulse repetition period will be 100 ns or more. It can be seen, therefore, that the receiver searches for a narrow (˜1 ns) signal in a wide (>˜100 ns) window, and only detects that signal when the incoming and replica pulse trains are perfectly aligned. Consequently, the search may be quite time consuming.
Once the phase offset which results in alignment of the local replica pulse train and the incoming pulse train at a particular time has been determined, the receiver applies that offset to the local replica pulse train, so that the sampling gate is open when the next incoming pulse arrives at the receiver.
Since coherent receivers receive a train of coherent pulses, they are able to perform a more sophisticated analysis of the incoming signal than non-coherent receivers. However, coherent receivers take time to initially lock on to the transmitted signal. They also suffer from mismatches between the frequencies of the local clocks at the transmitter and receiver which generate the transmitted signal and replica signal respectively.
There is thus a need for an improved UWB receiver which is able to perform a more sophisticated analysis of the incoming signal without increasing the time taken to lock onto the signal, or suffering from mismatches between the clocking at either end of the communication channel.